The following monthly COP figures for space heating and domestic hot water (DHW) are specifically for the periods of time that the heat pump is running. This shows higher COP's in the summer than the winter as we would expect given higher outside temperatures. The combined COP chart includes standby consumption and we can see that it drags down the summer COP's.
The average annual COP of 4.04 is very slightly higher than both 2019 and 2020 figures.
The following charts show the electricity consumption of the heat pump separated out again by space heating, hot water and standby. Standby consumption increases in summer as the heat pump spends a lot more hours on standby.
In the 2019 and 2020 reviews I compared our heating consumption in those years to the modelled heat demand from a SAP based energy assessment. I have since spent a bit more time looking at the original assessment assumptions and realise that space heating demand prediction given by SAP is highly dependent not just on the target temperature in the livingarea e.g 21C and the fabric assumptions but also the number of off hours assumed, the extent of the living area and the responsivness of the heating system. If we keep the target temperature the same at 21C the space heating demand prediction can be anywhere from a minimum of 8,558 kWh up to a maximum of 12,215 kWh depending on these non-fabric assumptions.
This makes it hard to give a single number for what the assessment suggests the space heating demand should be. It seems a fairer comparison to take the measured internal and external temperatures for each year, enter these in the SAP model to obtain a prediction for space heating demand and then compare that figure with the measured space heating demand. Here are the results of this comparison:
Predicted heat demand for 2019 based on measured internal and external temps: 7057 kWh, the actual space heat supplied was 5288 kWh = 25% less than the prediction.
Predicted heat demand for 2020 based on measured internal and external temps: 8286 kWh, the actual space heat supplied was 6011 kWh = 27% less than the prediction.
Calculating heat loss factors in W/K
Another approach to comparing modelled fabric performance with measurement is to calculate and compare heat loss factors. I go into the detail of how I do this in the 2020 blog post. It's again not quite possible to make a precise comparison as I still have to make assumptions for heat gains from water, body heat and solar gains. But it's interesting to try nonetheless.
The chart on the left shows the resulting monthly heat loss factor values, varying between 131 and 243 W/K and an average of 192 W/K. The average is only 6.3% lower than the heat loss factor calculated using the SAP assessment as shown in the graphic on the right. This is quite a bit less than last year where the difference was 15%.
The data for much of the analysis in this post was collected using our OpenEnergyMonitor monitoring system, I've written a more detailed post on the monitoring setup here: Monitoring. Here are the exported monthly temperature averages and consumption values for 2021:
See appendix of 2020 post for discussion of carbon intensity and carbon savings discussion.